Various methods for producing a polyimide powder have heretofore been proposed. One of the methods is a method comprising producing a high molecular weight polyamide acid by reacting an aromatic tetracarboxylic dianhydride and an aromatic diamino compound in an organic polar solvent such as N-methyl-2-pyrrolidone at a relatively low temperature, adding the resulting high molecular weight polyamide acid to a poor solvent therefor such as water, toluene or hexane, to precipitate a polyamide acid fine powder, removing the solvent, separating the fine polyamide acid by filtration and heat drying the fine polyamide acid at a temperature of from 200.degree. to 300.degree. C. to obtain a polyimide powder.
However, the polyimide powder obtained by the above proposed method has the problems that the particle size thereof is scattered and the particle size distribution is extremely broad, and more particularly, the polyimide powder obtained by using water as a precipitant does not form a high molecular weight polyimide since the amide acid produced is hydrolyzed by contacting water, causing a problem in heat resistance. In addition, another problem is that a molding made from such polyimide powder obtained by compression molding under heating has particularly poor mechanical strength.
Recently, a novel method has been proposed as disclosed, for example, in Japanese Patent Application (OPI) No. 200452/82 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). According to this method, polyamide acid is not precipitated in the form of fine particles as described above, but polyimide is directly precipitated in an organic solvent in the form of a polyimide wherein reaction has further proceeded. This method comprises dissolving substantially equimolar amounts of a biphenyltetracarboxylic acid component and an aromatic diamine component in a specific polyimide-insoluble and high boiling amide-based solvent at a temperature of 155.degree. C. or less to prepare a homogeneous solution having a low rotating viscosity, and heating the resulting solution to a temperature of from 160.degree. to 300.degree. C. in a short period of time with stirring and maintaining the solution at the same temperature to precipitate a powder-like aromatic polyimide having a relatively low molecular weight.
This proposed method has been provided to produce a polyimide powder directly by precipitating the polyimide without employing the two steps of producing a high molecular weight polyamide acid and then heating an organic polar solvent solution of the polyamide acid to a high temperature to imidate the polyamide acid and precipitate the polyimide as a powder. In short, this method has been intended to shorten the manufacturing time.
However, the above proposed method also has the problem that the particle size of the polyimide powder is scattered and the particle size distribution is very broad, since in this method a high molecular weight polyamide acid is not previously produced, rather, the polyimide powder is directly precipitated by rapidly heating to a high temperature of from 160.degree. to 300.degree. C. before the high molecular weight polyamide acid is produced, and as a result, a molding made from the polyimide powder by compression molding under heating (hereinafter referred to as "heat-press molding")also has poor mechanical strength. In addition, the molding is slightly inferior in heat resistance to that made from the above-described conventional polyimide powder produced through polyamide acid.
Thus, a polyimide powder which can provide a molding having excellent strength and heat resistance by heat-press molding has been not obtained by any conventional methods. Improvement thereof has been strongly demanded.